Superantigens (SAGs) comprise a class of toxins that elicit massive T cell proliferation through simultaneous interaction with major histocompatibility complex (MHC) and T cell receptor (TCR) molecules. SAGs have been implicated in the pathogenesis of a number of human diseases, including toxic shock syndrome, food poisoning and several autoimmune disorders, thought to be the result of the stimulation of large numbers of T cells and their subsequent release of inordinate levels of pyrogenic and inflammatory cytokines. Due to their extreme virulence and relative ease with which they could be produced and dispersed throughout a population, SAGs represent credible candidates for biological weapons of mass destruction. Indeed, SAGs have been identified as Category B Agents of Bioterrorism by the Centers for Disease Control and Prevention. The development of therapeutics against SAG-induced disease, against which no drug or vaccine exists, is therefore one important facet of an overall national defense against bioterrorism. We propose to develop a group of engineered TCR fragments with markedly improved affinity for SAGs that will specifically abrogate interactions between SAGs and their cell surface ligands in vivo, thereby inhibiting the pyrogenic cascade in its initial stages and curtailing SAG-induced disease. We will create these SAG-TCR interaction antagonists following a step-wise approach. Milestones along the development pathway for potential therapeutics of SAG-induced disease include (1) structure-function analysis of SAG-TCR interactions, (2) engineering of affinity matured TCR beta domain antagonists, (3) analysis of the in vivo efficacy of antagonists, (4) modification of antagonists to improve the in vivo efficacy, and (5) development of broad spectrum SAG-TCR antagonists. We will focus our initial efforts on guiding an antagonist that has been developed to inhibit interactions between SEC3 and the mouse TCR Vbeta8.2 domain through this development pathway, not only to produce an effective anti-SEC3 therapeutic but also as a general proof-of-principle exercise for the design of anti-SAG biologics. With knowledge gained from studies involving this engineered antagonist, we will shift our focus to targeted antagonism of the numerous hVbeta2.1-specific SAGs by analogous methods.